Photonic integrated circuits (PICs) are known in which various optical devices are provided on a common semiconductor substrate and interconnected or optically coupled with one another by waveguides. Such devices may include lasers, and in the case of a coherent receiver that receives polarization multiplexed optical signals, photodiodes and 90 degree optical hybrids circuits. Preferably, PICs should have a high density, so that more devices can be integrated on a given die, die size is reduced, and more die can be yielded from a given wafer. However, when integrating multiple devices on a substrate, waveguides are often laid out such that they cross one another in order to route optical signals from one device to another in a spatially efficient manner. Such waveguide crossings, however, may introduce unacceptably high optical loss and cross talk, which can impair optical signal quality and introduce transmission errors.
Accordingly, there is a need for a PIC, such as a coherent receiver PIC that receives polarization multiplexed optical signals, wherein the PIC has a simple, compact design. This design preferably has few or no waveguide crossings, can scale linearly, and can simply incorporate additional receiver circuitry, without additional complexity.